Invading without war ???

[Grisu]

I can't believe it, those bloody eoladi just invaded me and didn't even declare war. I'm used to frequent attacks on my scouts when I'm not on good terms with the race in question, but an all-out invasion is completely new to me. their task force just appeared near one of my frontier planet, did a short bombardement and then invaded and took control of my planet

ah well, my dreadnaughts are on the way to right this wrong

 

[Lucky]

Yeah, that seems to be a bug/feature of the game. You can do that yourself, too. This way you can quickly take some planets while having only to deal with sector defense forces and not his whole fleet.
But be aware, usually they strike back where you least expect it.